1. Field of the Invention
The present invention relates to the field of helicopter control mechanisms and more particularly relates to rotor blade pitch control mechanisms.
2. Description of the Prior Art
The prior art control systems for the cyclic and collective pitch of helicopter blades, typically involve a series of linkages and control rods for transmitting a force or displacement to the rotor blade pitch control horns to collectively control the pitch of all the blades or individually of any one blade.
Consider for example, the prior art system shown in FIG. 1 and briefly described here. FIG. 1 shows in diagramatic and simplified view the control linkages for a two blade rotor system depicted in a simplified form in isolation of all other elements of the helicopter. Blades 10 are controlled through conventional rotor blade pitch control horns 12, which when angularly displaced, will rotate blades 10 and thus alter the pitch of each corresponding blade. Each pitch control horn 12 is connected to an adjustable upper link 14 which is coupled at its opposing end to a corresponding rotating rocker arm 16. Rotating rocker arm 16 pivots about an axis perpendicular to the longitudinal axis of drive shaft 18. The opposing end of each rotating rocker arm 16 is then coupled to an adjustable lower link 20 which in turn is connected to an upper swashplate which is rotatable and tiltable with respect to the longitudinal axis of drive shaft 18. Upper swashplate 22 rides on a lower swashplate 24 which is mounted on gimbles and is allowed to tilt, but cannot rotate or move vertically. Lower swashplate 24 is controlled by connection through stationary and fixed linkage rods 26 to a control stick 28. Control stick 28 is movable in any direction and thus can impart through linkage rods 26 an arbitrarily selected inclination to fixed, nonrotatable lower swashplate 24.
Thus, control is effected by appropriately positioning control stick 28 to determine a fixed angular orientation for lower swashplate 24 which in turn is then transmitted to rotating upper swashplate 22 through lower links 20, to rocker arms 16, and then through upper links 14 to pitch control horns 12. By this means, the position of control stick 28 individually adjusts the pitch of each blade 10 of the helicopter rotor system consistent with the constraints and relationships imposed upon each of the blades by virtue of their common coupling through upper and lower swashplates 22 and 24.
The pilot also has a collective pitch lever 30 which is similarly connected through a linkage system, generally denoted by a reference numeral 32, to a collective pitch sleeve, vertical positioning mechanism 34 slidably connected to a collective pitch sleeve 36. Collective pitch sleeve 36 rotates in synchronism with drive shaft 18 and is vertically disposable thereon in parallel to the longitudinal axis of drive shaft 18. Thus, as collective pitch sleeve 36 is raised or lowered by movement of collective pitch sleeve vertical positioning mechanism 34, rotating rocker arms 16 are also moved vertically along the longitudinal axis of drive shaft 18 to collectively displace each of the corresponding pitch control horns 12 by an equal amount.
Such typical prior art systems are characterized by a number of substantial drawbacks. Firstly, the number of parts, connections, linkages and adjustments which are included within the pitch control system, even when illustrated in a highly simplified diagramatic view such as shown in FIG. 1, is large.
Secondly, a reactive force is applied to blades 10 whenever the pitch is changed and is maintained thereon thereby tending to return blades 10 to zero pitch. This reactive force is transmitted through pitch control horns 12 and the linkage system of FIG. 1 and is felt at a pilot's control pitch lever 30 and control stick 28 as a reverse feedback control force and motion.
Thirdly, all of the control elements as illustrated in FIG. 1 are clearly exterior to drive shaft 18. These elements rotate with shaft 18 thereby contributing substantial air drag not only when the helicopter is in flight as a whole, but when hovering as well.
Fourthly, complex and multiple linkages such as shown in typical prior art systems of FIG. 1 are prone to cumulative error or control lag and looseness such that a small movement in the pilot's control stick 28 or pitch lever 30 does not precisely result in any equal amount of displacement in the desired direction in pitch control horns 12. The result is that the control systems of the prior art have a loose feel, tend to be inaccurate in response and variable between one helicopter and the next, even when of identical design, due to small assembly differences and differences in wear.
Finally, such prior art systems are highly vulnerable to damage, particularly in military rotorcraft where even slight damage to any portion of the control linkage can materially interfere with control or result in a stress failure.
Therefore, what is needed is a control system which reduces the number of parts and connections thereby increasing the reliability and lessening the probability of mechanical breakdown, and which system provides an accurate, stiff, precise blade control system, with reduced air drag in the rotor system and which insulates the pilot's controls from feedback motion and feedback control forces.